The present invention relates to extrusion dies and a method for making a cellular honeycomb structure, particularly, ceramic honeycomb structures for use as catalytic converters in the exhaust stream of internal combustion engines. While the invention may be used in other types of dies, it is especially suited for use in extrusion dies for manufacturing thin-walled honeycomb structures from an extrudable material, such as ceramic, and will be particularly described in that connection.
As the use of ceramic honeycomb structures in catalytic systems has increased, so has the need for efficient ceramic honeycomb structures. The efficiency of a catalytic converter is a function of the surface area available to the exhaust stream for interaction with the noxious output of internal combustion engines. Hence, by increasing the cell density of the structure or by increasing the surface area per cell, the efficiency of a catalytic converter can be significantly enhanced. One way to increase the cell density of a honeycomb structure is to increase the number of the discharge/outlet slots in an extrusion die. Typically, an extrusion die includes feed holes/feed channels that originate at one end, and which are longitudinally aligned and in communication with a criss-crossing grid of discharge slots that form pins/pegs terminating at an opposite end, that is on an outlet face of the die. The specific layout and construction of the discharge/outlets in an extrusion die dictates the size and shape of the cells in a honeycomb structure that is extruded from that die. By increasing the number of the discharge/outlets, the cell density of the honeycomb structure is increased. However, to ensure the formation of a fully knitted honeycomb structure, it becomes necessary to increase the number of feed holes to compensate for the increase in the number of discharge/outlets, so that proper batch flow of an extrudable material through an extrusion die is maintained.
Another way to increase the efficiency of a catalytic converter is to increase the surface area of a cellular honeycomb structure. One way of increasing surface area is to form protrusions or extension surfaces in a cell of the honeycomb structure. Protrusions can be formed by compound slot extrusion dies in instances where the extrudable material being delivered through the partial discharge/outlets of the die fails to fully knit, thus, resulting in a break in a cell wall. However, the accuracy of the size and the variability of the protrusions are difficult to control precisely. Alternatively, shorter length partial discharge/outlets can be machined into the face of the pins/pegs in an existing extrusion die to accurately control the protrusion lengths. However, this would require cutting the face of the pins/pegs by an expensive machining process.
In light of the foregoing, it is desirable to provide an extrusion die that can be used to form a honeycomb structure with a greater surface area. In addition, it is desirable to provide a method for modifying an existing extrusion die in combination with a device that can be easily attached to the existing extrusion die that can produce accurate protrusions in a honeycomb structure. Furthermore, it is desirable to provide an affordable method of making extrusions dies for forming honeycomb structures with greater surface area. Also, it is desirable to provide a method for making a cellular honeycomb structure where the cell density can be varied within the structure.
Accordingly, the present invention is directed to a composite extrusion die and a method of manufacturing a composite extrusion die that forms a cellular honeycomb structure with a greater surface area that substantially obviates one or more of the limitations and disadvantages of the related art. The principal advantage of the present invention is the provision of an arrangement that overcomes the limitations and disadvantages of the described prior arrangements. Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the apparatus particularly pointed out in the written description and claims hereof as well as the appended drawings.
In accordance with a preferred embodiment of the present invention, the composite extrusion die is for forming a cellular honeycomb structure having at least one finned protrusion extending from at least one cell wall of the cellular honeycomb structure. The composite extrusion die comprises a primary extrusion die body having an inlet face and an outlet face, and at least a first insert member that is secured into a first intersection formed by a pair of intersecting slots at the outlet face of the primary extrusion die body. More preferably, a plurality of insert members are secured into a plurality of intersections, each intersection being formed by a pair of intersecting slots at the outlet face of the primary extrusion die body, where each of the insert members is adapted to fit into each of the respective intersections of the die body and where the size and shape of the insert members and the positioning of the insert members into the intersections cuts off the flow path of an extrudable material through the respective intersections whereby the cellular honeycomb structure that is extruded from the composite extrusion die comprises a plurality of finned protrusions extending from at least one of the cell walls of the honeycomb structure corresponding to the intersection into which the insert member is secured in the die body. The invention also provides a method for increasing the surface area of a cellular honeycomb structure by producing a protrusion in at least one cell wall of a honeycomb structure, preferably, producing a protrusion in each cell wall of the honeycomb structure. Further, the size and variability of these protrusions is controlled by securing an insert member into at least one intersection, preferably, a plurality of intersections at an outlet face of the extrusion die, where each of the insert members is designed to either partially or completely obstruct the intersection. Further, in a preferred embodiment, each of the insert members is secured in place by using an adhesive or epoxy. In the invention, each of the intersections is formed by two discharge/outlet slots that intersect each other at a right angle and is defined by four pins/pegs on the outlet face of the extrusion die with each of the insert members being securely inserted into one of the intersections. Further, in a preferred embodiment, each of the intersections into which an insert member is inserted is not directly connected to a feed channel. In a preferred embodiment, the method includes inserting a plurality of insert members that are either in the form of a fixed cross-shaped or a pair of interlocking plates or a pin that is securely inserted into the respective intersections of the extrusion die, whereby, the batch flow path can be defined and, hence, the surface area of each cell that makes up a honeycomb structure can be varied as desired. Further, in one embodiment, all of the intersecting outlet slots are of the same slot width and slot depth, whereas, in another embodiment, alternating intersecting outlet slots have the same slot width and slot depth, where a first array of the alternating intersecting outlet slots are primary outlet slots and where a second array of the alternating intersecting outlet slots are secondary outlet slots. Each of the secondary outlet slots are at least equal in width to each of the primary outlet slots, and each of the secondary outlet slots are smaller in depth than each of the primary outlet slots. In a preferred embodiment, each of the secondary outlet slots are larger in width than each of the primary outlet slots, and each of the secondary outlet slots are smaller in depth than each of the primary outlet slots.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the extrusion die is for making a cellular ceramic honeycomb structure used in catalytic systems. Furthermore, a method is provided for making the extrusion die and for making the honeycomb structure using the extrusion die of the present invention.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrating embodiments of the invention, and together with the description serve to explain the objects, advantages, and principles of the invention.